The invention relates to a process, an apparatus and a pressure reactor for the treatment of solids with pressurised liquid gases, in liquid ammonia.
A process of the type is described, for example, in the WO 96/3 04 11. With the known processes polysaccharides are brought into contact with pressurised liquid ammonia. During the subsequent pressure release the volume available to the polysaccharide/liquid ammonia system is enlarged explosion-like whilst reducing the pressure by at least 5 bar. By doing so an increased accessibility and reactivity of the treated polysaccharides can be obtained.
To obtain an as high as possible yield of treated solids in a short time, such processes are preferably carried out continuously. However, with the continuous process suitable steps must be taken to feed the solid into the pressure tank.
From the DE-27 14 993 a process is known for feeding fibrous lignose cellulose raw material into a pressurised tank. Here the raw material, before entering the pressure tank, is pre-compacted to a density of at least 0,72 g/cm3 and is then fed into the tank by a conveying screw. The pre-compacted raw material then acts as a plug which on passing the inlet opening of the pressure tank seals same off, so that the pressure in the tank can be maintained. As a result thereof it is possible to continuously feed cellulose to the tank.
However, this continuous process requires a great amount of apparatus on the feeding side. In addition, due to the high pressure that must be exerted on the cellulose to compact it, a disadvantageous change of the inherent properties of the cellulose is produced.
Furthermore, the known method requires a dwell time (approx. 4 minutes) of the cellulose in the pressure tank, which is unnecessarily long especially for the treatment with pressurised liquid ammonia, in particular under high pressure, since as is known, liquid ammonia diffuses into solids within a few seconds up to one minute and in doing so is uniformly distributed.
Moreover, a reliable and adequate sealing off of the apparatus when working with liquid ammonia under pressures of up to 40 bar, in particular at the lead-through of the drive shafts for the pressure tank and for the screw conveyor in the outlet part underneath the pressure tank, is problematical. The sealing off of the pressure reactor to the outside takes place, as indicated above, by the to be treated material itself. This may possibly be satisfactory in the case of very moist, plastically deformable solids, such as for example wood chips. With cellulose, guar splits, i.e. hard, small lentil-shaped seeds, as well as with minerals, e.g. zeolites or silicates, as solid, a sealing is, however, hardly possible.
Finally, the smallest possible size of the apparatus for carrying out the known process lies at a through-put of approximately 400 tons per year, which is much too large for specific fields of application.
From the U.S. Pat. No. 5,171,592 a process is known, with which the to be treated biomass is pressed by means of a solids pump against a large valve so as to compact the material and press out the enclosed air before the valve is opened to feed the biomass into the reactor. The reactor is provided with finger-like teeth on the inside wall of the reactor and on a rotating tool. The teeth have a hole for dosing liquid ammonia into the reactor. The reactor outlet is provided with a valve through which the ammonia-treated biomass can escape explosion-like into a collecting tank. The apparatus for carrying out the process is, therefore, relatively complex and not suitable for processes that are carried out under high pressure, seeing that rotating machine parts are located in the high-pressure area, which causes increased wear and susceptibility to problems of the known apparatus.
It is, therefore, the object of the present invention to indicate a process with which the described disadvantages of the state of the art are avoided, and with which at the same time the treated solid is obtained practically continuously. It is a further object of the invention to make available an apparatus suitable for carrying out the process and a respective pressure reactor, which stand out by a high availability and low maintenance costs.
These objects are achieved by the process for the treatment of solids with pressurised liquid gases, in particular liquid ammonia, according to claim 1, by the apparatus according to claim 10 and by the pressure reactor according to claim 45 or the process according to claim 55.
According to same, with the process according to the invention the to be treated solid is fed into a pressure reactor at atmospheric pressure, subsequent to which the pressurised liquid gas is fed to the pressure reactor and after a pre-set dwell time the resultant liquid gas/solid mixture is expanded explosion-like into an expansion tank, wherein at least two reactors are operated in a time-staggered manner.
With the process according to the invention it is possible to feed a pressurised liquid gas into a solid present in any form, and to treat it with the pressurised liquid gas for a pre-set time, without a drop in pressure occurring and without the to be treated solid itself being used for sealing functions so as to maintain the pressure. For this reason, at the start of the process the to be treated solid need not be compacted so heavily that its structure or morphology changes or is adversely affected. Accordingly, a rapid and uniform diffusion of the pressurised liquid gas in the to be treated solid is ensured.
At the start of the process according to the invention, in particular after a preceding preparation in the sense of a splitting up or comminuting, the to be treated solid can be pre-compacted to a desired bulk density or to a specific compacting degree. In this way the yield of treated solid per unit of space and time can be adapted to the requirements in question, for example with regard to the size of the apparatus. In this connection it must be borne in mind, however, that the compacting pressure is kept so low that no change in the inherent properties of the solid occurs.
The process according to the invention can be used for the treatment of cellulose, starch, gelatine, guar or wood chips and in general of polysaccharide-containing materials, but also for the treatment of minerals such as silicates and zeolites as well as of thermoplastic polymers. Because of its particularly good diffusion properties, ammonia has proved eminently suitable for use as liquid gas.
By the cyclic operation of at least two pressure reactors, a quasi-continuous treatment of the solid can be obtained, so that starting material can be made available practically uninterruptedly for further processing.
Preferably, the gas released during the explosion-like pressure release is recovered. The recovered gas can subsequently be fed back to the process in liquid form. This permits a particularly economic mode of operation, as in total only a small part of the used gas, which for example escapes into the atmosphere by unavoidable diffusion or evaporation, need be replaced.
The solid can be fed into the pressure reactor with the aid of dosing screws. By using this known, technically perfected conveying means, the process according to the invention can be implemented particularly reliably.
Alternatively, the solid can also be fed into the pressure reactor by a pneumatically operating conveying device. By this measure a particularly quick and specific feeding of the to be treated solid can be achieved.
Preferably, the to be treated solid is actively mixed with the liquid gas in the pressure reactor. This has the advantage that the liquid gas penetrates even more deeply into the solid and distributes itself even more homogeneously in same. Furthermore, by this additional measure the dwell time of the mixture liquid gas/solid in the reactor can be reduced and accordingly the throughput per pressure reactor can be increased. This measure is particularly expedient when the liquid gas has only moderately good diffusion properties or when a particularly high activation of the to be treated solid is required.
According to a preferred embodiment the opening and closing of the reactors and/or the feeding of the solid and/or of the liquid gas is controlled automatically. By combining the mentioned measures the entire process can practically be automated, so that the operating personnel only has to attend to monitoring and possibly maintenance duties of the apparatus.
If desired, together with the pressurised liquid gas a solid or liquid additive dissolved or dispersed therein can be fed into the reactor. In this way, together with the treatment with liquid gas, the solid can be dosed with other substances, for example to further increase its activation. In this case preferably ammonia is used as liquid gas, as numerous additives can easily be dissolved in same.
The liquid gas is preferably mixed with the solid or liquid additive before it enters the reactor. By doing so, a homogeneous solution or dispersion of the additive and the liquid gas can be produced.
The apparatus according to the invention for the treatment of solids with pressurised liquid gas comprises at least two parallel arranged pressure reactors for the alternating taking in of a solid and a liquid gas, which each have inlet and outlet openings for the solid provided with shut-off elements as well as each at least one inlet opening for the liquid gas, at least one expansion tank which is connected to the respective pressure reactors, and conveying means for feeding the solid as well as the liquid gas.
With the apparatus according to the invention a quasi-continuous operation during the treatment of a solid with pressurised liquid gas can be realised. The inside of the reactor can be sealed off from the atmosphere in a simple manner by the shut-off elements, so that no complex structural or process-technical measures are required to maintain the pressure during the carrying out of the process.
The shut-off elements of the pressure reactors preferably are ball valves. These have the advantage that they are technically matured and tested. They can be used for millions of cycles without material fatigue symptoms occurring. Accordingly, at an assumed cycle time of 1 minute and 8000 operating hours per year, the valves need not be replaced for several years. Also, special valves of ceramic or with hardened surfaces are commercially obtainable, which withstand particularly high stresses.
According to a specially preferred embodiment, the pressure reactors are upright tubular cylinders. In this way the filling with comminuted and pourable solids is facilitated, as here the force of gravity aids the feeding in of the solid. These tubular cylinders can be manufactured particularly easily and their number can be chosen in accordance with the optimum dwell time for the to be treated solids. The cycle frequency and accordingly the number of tubular cylinders to be used depends, therefore, on the filling time of each tubular cylinder. At an assumed filling time of approximately one minute, for example, with eight cylinders which each have a capacity of 8 kg, a throughput of 2,4 tons/hour can be obtained. To avoid material losses during the filling, the inlet of the reactor may be widened upwards in the form of a hopper.
Depending on whether a technically particularly simple solution is desired for feeding the solid or whether above all a high feeding speed is required, conveying screws or pneumatic conveyors can be chosen as conveying means for the solid.
As has already been mentioned, means can be provided for the automated control of the process operations, in particular the feeding of the solid and/or the liquid gas and/or the cyclic actuation of the shut-off elements of the pressure reactor.
According to another advantageous embodiment, the pressure reactors are equipped with external heating. By means thereof, in conjunction with a suitable regulating, a constant temperature can be ensured inside each reactor.
The reactors preferably have several openings each for the feeding in of the liquid gas. In this way the liquid gas can be distributed particularly finely over the entire inside of the reactor and accordingly over the surface of the solid.
With a view to a better distribution of the liquid gas over the solid or into the inside of same, the reactors may also be designed as mixers. In this case, however, technically complicated measures must be provided for a secure sealing off of the reactor.
As will still be explained in greater detail in the following, the shut-off elements at the inlet of each pressure reactor may be designed as a sluice system. By doing so the impermeability of the reactor can be additionally increased or secured.
The apparatus according to the invention preferably comprises a compacting device, which compacts the solid, in particular cellulose, in the pressure reactor. By compacting the comminuted cellulose in the pressure reactor, a higher space/time yield and, accordingly, comparatively smaller apparatus sizes can be achieved. By compacting the solid, e.g. the comminuted cellulose, to a specific pressing effect or filling degree, a filling of the pressure reactor is advantageously obtained within relatively narrow limits. A complicated and expensive gravimetric dosing system can, therefore, fall away.
The compacting device preferably has a compacting piston, which moves in a cylindrical reactor chamber (34, 50; 81) or dosing chamber and compacts the solid. By means of the piston a gentle compacting of the solid is obtained.
The compacting piston preferably has one or several ducts which extend liquidpassable from an underside to an upper side of the compacting piston. By the perforation of the piston disturbing propellant gas can escape from the reaction chamber during the compacting and an accurately reproducible filling degree of the solid is obtained.
The compacting piston preferably moves the compacted solid from a compacting chamber into a reaction chamber of the pressure reactor. As a result thereof extra means for charging the reaction chamber can be dispensed with.
The compacting piston preferably can be locked or at least held in a position of its maximum piston stroke, as a result of which additional shut-off means for the separation between compacting and reaction operation can be dispensed with. Furthermore, by doing so different filling levels and filling volumes of the solid in the reactor chamber can be realised.
Preferably, a solids feeding device is provided which feeds the solid to a pressure reactor or a group of pressure reactors, wherein the solids feeding device has a conveying propeller which ensures a continuous feeding of the solid.
Preferably, a preparation device is provided for the solid, which prepares the solid before it is fed to the pressure reactor or the pressure reactors, as a result of which the reaction time in the pressure reactor is reduced. Preferably, as preparation device a heating device is used which heats the solid.
Preferably, a heat carrier fluid, which is heated by the heating device, is fed to the solid as a result of which a homogeneous heating of the solid during the preparation is ensured.
Preferably, liquid or gaseous ammonia is used as heat carrier fluid, as a result of which the absorption of liquid ammonia in the pressure reactor is improved further and even shorter reaction times are obtained.
Preferably, the heating device heats the ammonia to a temperature of approximately 100xc2x0 C., by which favourable conditions are produced for the ammonia absorption in the pressure reactor.
Preferably, the heat carrier fluid circulates in a heating circuit, by which energy and fluid are saved.
Preferably, the solid is heated in a conveying screw, by which an accurate reaction time of the heat carrier fluid on the solid and accordingly a constant and accurate heating of the solid can be realised.
The process according to the invention preferably comprises the following pressure reactor phases, wherein a pressure reactor is charged with a solid during a filling phase, wherein subsequently during a compacting phase the solid is compacted in the pressure reactor, wherein then during a reaction phase liquid gas is fed into the compacted solid in the pressure reactor, by which a liquid/solid mixture is produced, and wherein then the liquid/solid mixture is expanded explosion-like into an expansion tank. By the compacting step, so to speak a uniform, accurate and cost-saving dosing of the solid and an economical operation can be obtained due to a high space/time yield, compared to processes without compacting step.
As liquid gas, a mixture of ammonia and urea may, for example, be fed to the cellulose in the pressure reactor, so as to produce cellulose carbamate with the process according to the invention, the apparatus according to the invention and the pressure reactor according to the invention, respectively.
Still before be reaction phase, the filling phase and the subsequent compacting phase of the solid in the pressure reactor can be repeated one or several times, to obtain a pre-set compacting degree and/or filling level of the solid in the pressure reactor.
Several pressure reactors can be operated simultaneously, time staggered, in the aforementioned phases, so as to obtain a quasi-continuous process with a high yield.
The pressure reactor according to the invention for the treatment of a solid, in particular cellulose, with a pressurised gas or a liquid, in particular liquid ammonia, optionally together with urea, comprises a compacting device which presses together the solid filled into the pressure reactor so as to compact it, by which the advantages already indicated above are obtained.
The process according to the invention for the treatment of a solid, in particular cellulose, with a pressurised gas or a liquid, in particular liquid ammonia, is carried out with the following steps: a pressure reactor is filled with a solid during a filling phase, subsequently during a compacting phase the solid is compacted in the pressure reactor, then during a reaction phase the gas or the liquid is fed to the compacted solid in the pressure reactor, by which a liquid/solid mixture or gas/solid mixture is produced, and then the mixture is expanded explosion-like into an expansion tank.
Further advantageous further developments of the present invention can be noted from the sub-claims.